Investigating and Exploiting a New Strengthening Mechanism Found in Biomineralized Composites

Auburn University, Auburn AL

Investigators

Abstract

Many living organisms form biomineralized composites assembled in hierarchical architectures for protective purposes or structural support (sea shells, bone, antler, dentin, etc.). These natural composites often have excellent mechanical properties in contrast to their non-biomineralized forms. This work leverages a recent discovery by the PI of a new strengthening mechanism found in Abalone shells. In particular, the influence of periodic growth interruptions found in the shells from seasonal changes in its natural habitat, analogous to tree rings. Preliminary results show that the presence of these layers as well as their thickness and periodicity appear to impart added strength to the shell. This work is aimed at determining the fundamental mechanism by which these growth interruptions impart enhanced behavior to the multilayer, composite shell. In particular, it will culture the Abalone in the lab and use water temperature to trigger their metabolic system to generate growth interruptions. The material properties of these interruptions and the nacre will be characterized (1) on an individual basis, (2) as a two component composite and (3) as a multilayer composite. Ascertaining how these organisms utilize an architecture that combines different material properties in the layers would give engineers with new knowledge on material strength. It also will provide tools to design new classes of composite architectures possessing enhanced properties and performance. These undoubtedly would impact applications ranging from mechanically protective armor to stronger and lighter structural supports and even to enhanced durability of asphalt roads. The interdisciplinary nature of the project lies at the interface between physics, biology and engineering, which is an ideal setting for the education and development of students in technologies and techniques that have a rapidly growing demand. This project is aimed at enabling transformative advancements in the strength and toughness of multilayer composites by leveraging architectures and material property combinations found in biomineralized composites. It focusses on the role that growth interruptions play in the nacre structure. These interruptions occur when the organism's metabolic processes are interrupted from seasonal changes in its natural habitat, analogous to tree rings. This work will involve culturing Abalone in the laboratory setting, subjecting them to environmental parameters that trigger their metabolic system to generate a growth interruption. Shell architectures will be modulated in this way to determine those characteristics (thickness, periodicity, etc.) that impart maximum strength and toughness. The primary intellectual merit contributions of this work involve: (1) providing an improved understanding and measurement of the individual material properties ((elastic modulus, Poisson's ratio, hardness, etc.) of the nacreous and growth interruption materials as well as their interplay in determining composite behavior; (2) providing architectural and combinatory material guidelines for optimizing the response of multi-layered composites for mechanically protective applications based upon Poisson's ratio mismatch; and (3) enabling an assessment as to whether natural selection in mollusk organisms has fully maximized the toughening mechanisms that these multilayer composite architectures provide.